What is the effect of negative skin friction on the load carrying capa...
Negative skin friction: It is usually a downward shear drag acting on a pile or pile group because of the downward movement of surrounding soil relative to the piles. Such a force can exist under one of the following conditions:
- If a fill of clay soil is placed over a granular soil layer into which a pile is driven, the fill will exert a downward drag force on the pile due to consolidation.
- If a fill of granular soil is placed over a layer of soft clay, it will induce the process of consolidation in the clay layer and thus exert a downward drag on the pile.
- Lowering of the water table will increase the vertical effective stress on the soil at any depth, so it can induce negative skin friction on piles.
Now the axial capacity of a pile is the summation of the upward reaction due to bearing at the base and net upward skin frictional resistance. As the negative skin friction (acting downward) lowers the net skin resistance, it in turn reduces the axial capacity of piles.
A schematic diagram of negative skin friction on piles is shown below:

∴ Negative skin friction on a pile under vertical compressive load acts downwards and reduces the load-carrying capacity.
What is the effect of negative skin friction on the load carrying capa...
Effects of Negative Skin Friction on Load Carrying Capacity of Pile Foundations
Decreases the Load Carrying Capacity:
Negative skin friction occurs when the soil surrounding the pile settles or consolidates, creating downward forces on the pile. This frictional force is opposite to the direction of the applied load and reduces the effective load carrying capacity of the pile foundation.
Mechanism of Negative Skin Friction:
When a pile is driven into soil that consolidates or settles over time, the pile may experience negative skin friction. This occurs because the settlement of the surrounding soil causes the pile to be restrained, leading to downward forces along the length of the pile. These forces can reduce the ability of the pile to carry loads effectively.
Reduction in Pile Capacity:
Negative skin friction can significantly reduce the load carrying capacity of pile foundations. The downward forces acting on the pile can increase the effective stresses within the soil, leading to a decrease in the ultimate capacity of the pile. This reduction in capacity can affect the overall stability and performance of the structure supported by the pile foundation.
Importance of Mitigation:
To mitigate the effects of negative skin friction, various techniques can be employed during the design and construction of pile foundations. These techniques may include the use of pile jackets, lubricants, or geotextiles to reduce the frictional forces acting on the pile. Proper consideration and mitigation of negative skin friction are essential to ensure the long-term performance and stability of pile-supported structures.